1. Field of the Invention
The present invention relates to an antenna device which uses a shielded cable having flexibility, which is applicable to portable electronic devices such as portable AV equipment and mobile telephones.
2. Description of the Related Art
In the field of consumer electronic products, there is AV equipment typified by portable sound reproduction equipment, and so on, and there is also a case where the sound of the equipment itself is heard through earphones (including headphones) using a coaxial cable.
In recent years, a portable television receiver has been also developed, and there is also a case where the sound thereof is heard through earphones. A cable for earphones is formed by a shielded cable and also used in the transmission of a high-frequency signal of a receiving antenna or the like.
In this manner, the technology of using an earphones cable as an antenna has been proposed.
This kind of cable is used in order to transmit an audio signal (low frequency band), and, for example, in a case where it is used for an application to antennas of VHF and UHF, there is a case where it is not suitable due to a large loss in a high-frequency signal.
Also, in the case of an ordinary coaxial cable called 3C-2V or 5C-2V for a high-frequency signal, although by optimizing high-frequency design, a high-frequency transmission characteristic could become excellent, there was a problem in that it is thick, heavy, and low in flexibility or tensile properties and durability performance at a movable portion is very poor.
Therefore, the applicant proposed a shielded cable which can be used in a movable portion like an earphone cable and transmit a direct-current signal (refers to Japanese Unexamined Patent Application Publication No. 2006-164830).
Since as a principal conductor of the shielded cable, an ordinary annealed copper wire can be used, and also, as a reinforcing filament body, a general-purpose filament body can be used, the cable can be manufactured at a low price.
Also, by using a filament body of a material, which is low in rigidity, but high in tensile strength properties, for a reinforcing filament body of the shielded cable, it becomes possible to prevent occurrence of the breaking of wire by increasing tensile strength without lowering a bending property and flexibility, and also, secure a given electric characteristic.
Also, as an example of an antenna using a coaxial cable, a so-called sleeve antenna is proposed (for example, refers to FIG. 1 of Japanese Unexamined Patent Application Publication No. 2003-249817 and FIG. 1 of Japanese Unexamined Patent Application Publication No. 2003-8333).
In the case of the sleeve antenna, the antenna has a structure in which a signal is transmitted by a coaxial cable and an antenna element is disposed at the leading end of the coaxial cable.
Particularly noteworthy is a folded structure of a ground GND, which is called a sleeve.
The sleeve antenna blocks an electric current, which is carried by an outer covering of the cable, by increasing impedance in terms of high-frequency by the folded structure of the sleeve.